1. Field of the Invention
The invention generally relates to a synchronization data detection unit and a method for detecting predetermined synchronization data, and in particular to a receiver and a receiving method in a wireless local area network (WLAN) communication system.
2. Description of the Related Art
In a communication system such as a wireless local area network (WLAN) system, it is important for a receiver to be synchronized to the transmitter so that messages can successfully be exchanged between the transmitter and the receiver. A wireless local area network system is a flexible data communication system implemented as an extension to or an alternative for a wired LAN. WLAN systems transmit and receive data over the air using radio frequency or infrared technology to minimize the need for wired connections. Thus, WLAN systems combine data connectivity with user mobility.
Most WLAN systems use spread spectrum technology, a wide-band radio frequency technique developed for use in a reliable and secure communication system. The spread spectrum technology is designed to trade-off band-width efficiency for reliability, integrity and security. Two types of spread spectrum radio systems are frequently used: frequency hoping and direct sequence systems.
In direct sequence spread spectrum systems, spreading is achieved by encoding each data bit using a code word or symbol that has a much higher frequency and information bit rate. The resultant “spreading” of the signal across a wider frequency bandwidth results in a comparatively lower power spectrum density, so that other communication systems are less likely to suffer interference from the device that transmits the direct sequence spread spectrum signal. Direct sequence spread spectrum employs a pseudo random noise code word known to the transmitter and receiver to spread the data. The code word consists of a sequence of “chips” that are multiplied by (or exclusive-ORed) with the information bits to be transmitted. Many wireless networks conform the IEEE 802.11 standard which employs the well-known Barker code word to encode and spread the data. The Barker code word consists of a predefined sequence of eleven chips. One entire Barker code word sequence is transmitted at the time period occupied by an information-containing symbol.
To allow higher data rate transmissions, the IEEE 802.11 standard was extended to IEEE 802.11b. In addition to the 11-bit Barker chip, the 802.11b standard uses an 8-bit complementary code keying (CCK) algorithm for high data rate transmission.
The data transfer rate may also be improved above the symbol rate by employing higher order modulation techniques, including quadrature phase-shift keying (QPSK) modulation. According to such modulation techniques, each bit is represented by a higher number of possible phases. The transmitter therefore generates two signals, the first signal is called the “in-phase” (I) signal or “I channel” and the second signal is called the “quadrature” (Q) signal or “Q channel” for a 90 degree phase-shifted sinusoidal carrier at the same frequency.
The IEEE 802.11 standard for wireless LANs using direct sequence spread spectrum techniques employ a training preamble to train a receiver to a transmitter. Each transmitted data message comprises an initial training preamble followed by a data field. The preamble includes a synchronization field to ensure that the receiver can perform the necessary operations for synchronization. For the preamble length, two options have been defined, namely a long and a short preamble. All compliant 802.11b systems have to support the long preamble. The short preamble option is provided in the standard to improve the efficiency of the network throughput when transmitting special data such as voice or video. The synchronization field of a preamble consists of 128 one bits for a long preamble and 56 zero bits for a short preamble.
A receiver detects the synchronization symbols and aligns the receivers internal clock with the symbols in the synchronization field in order to establish a fixed reference time frame with which it interprets the fields in the transmission frame structure following the preamble. The preamble, including the synchronization field, is transmitted with the start of every message (data packet).
The purpose of a preamble detection unit is to continuously monitor the incoming signal for the preamble and to indicate if the preamble has been detected. The boundaries between consecutive Barker symbols or CCK symbols are determined and the forwarding of the symbols is to be synchronized to the receiver's processing schedule. Based on the preamble detection and a timing offset between a symbol arrival and a processing schedule of the following modules, the incoming signal is synchronized to the receivers processing schedule.
Referring now to FIG. 1, a detection process for detecting a preamble in a communication signal is illustrated. A preamble detection step 101 is performed after receiving a communication signal 100 and before subjecting the received communication signal to further processing, in particular to descrambling 102.
The configuration of a conventional preamble detector 200 is illustrated in FIG. 2. The received communication signal 201 consisting of an in-phase and a quadrature component is provided to preamble detector 200. In the preamble detector 200, the received communication signal 201 is first applied to a despreader 204, in particular a Barker matched filter (BMF). The despread communication signal is supplied to a demodulator (DEM) 205 for demodulating the despread communication signal. The demodulated signal consists of a sequence of “hard” decisions of the received bit sequence, i.e. each data value of the demodulated signal takes one of both possible binary values. The demodulated bit stream is monitored for detecting the predefined preamble data. Typically, a correlator (e.g. correlator 203) is used to detect the preamble. The correlator is essentially a matched filter for the preamble sequence. The correlator produces an output with a large magnitude when the preamble is present. Preamble detection is normally declared when the magnitude of the correlation exceeds a predefined threshold.
After preamble detection, the demodulated communication signal is applied to a (digital) descrambler (DDS) 206. An example of a prior art descrambler 300 (which may be simalar to DDS 206 of FIG. 2) is illustrated in FIG. 3. The incoming signal 301 a is supplied to delay blocks 304, 305 denoting a time delay of several units in accordance with a predefined descrambling rule. The delayed signals are fed back and combined using a multiplicator or exclusive-OR gate 306. The output is fed back to the incoming signal 301b and combined using a multiplier or exclusive-OR gate 303 to produce a descrambled output 302.
Synchronization data detecting units still have a number of problems. One problem is that noise may degrade the signal quality so that the synchronization unit, in particular the preamble detector, fails to declare a preamble even though a preamble is present in the received communication signal. Noise may also produce an output exceeding the threshold when an actual preamble is not present.